They were connected to the shell by intricate lines known as sutures—the complexity of which varied tremendously throughout this animal’s evolution. The many chambers of their shells likely ...

These pore-pairs are situated along the outer (adambulacral) margins of the ambulacral zone. The central suture line between the two columns of plates in each ambulacral zone is the perradial suture ...

Look for the sagittal suture – the squiggly line that runs the length of the skull – and note whether is it's completely fused. If it is, the remains are likely to be of someone older than 35.